A known molten bath-based smelting process is generally referred to as the HIsmelt process, is described in a considerable number of patents and patent applications in the name of the applicant.
Another molten bath-based smelting process is referred to hereinafter as the “HIsarna” process. The HIsarna process and apparatus are described in International application PCT/AU99/00884 (WO 00/022176) in the name of the applicant.
The HIsmelt and the HIsarna processes are associated particularly with producing molten iron from iron ore or another iron-containing material.
In the context of producing molten iron, the HIsmelt process includes the steps of:
(a) forming a bath of molten iron and slag in a smelting chamber of a smelting vessel;
(b) injecting into the bath: (i) iron ore, typically in the form of fines; and (ii) a solid carbonaceous material, typically coal, which acts as a reductant of the iron ore feed material and a source of energy; and
(c) smelting iron ore to iron in the bath.
The term “smelting” is herein understood to mean thermal processing wherein chemical reactions that reduce metal oxides take place to produce molten metal.
In the HIsmelt process solid feed materials in the form of metalliferous material and solid carbonaceous material are injected with a carrier gas into the molten bath through a number of lances which are inclined to the vertical so as to extend downwardly and inwardly through the side wall of the smelting vessel and into a lower region of the vessel so as to deliver at least part of the solid feed materials into the metal layer in the bottom of the smelting chamber. The solid feed materials and the carrier gas penetrate the molten bath and cause molten metal and/or slag to be projected into a space above the surface of the bath and form a transition zone. A blast of oxygen-containing gas, typically oxygen-enriched air or pure oxygen, is injected into an upper region of the smelting chamber of the vessel through a downwardly extending lance to cause post-combustion of reaction gases released from the molten bath in the upper region of the vessel. In the transition zone there is a favourable mass of ascending and thereafter descending droplets or splashes or streams of molten metal and/or slag which provide an effective medium to transfer to the bath the thermal energy generated by post-combusting reaction gases above the bath.
Typically, in the case of producing molten iron, when oxygen-enriched air is used, it is fed at a temperature of the order of 1200° C. and is generated in hot blast stoves. If technically pure cold oxygen is used, it is typically fed at or close to ambient temperature.
Off-gases resulting from the post-combustion of reaction gases in the smelting vessel are taken away from the upper region of the smelting vessel through an off-gas duct.
The smelting vessel includes refractory-lined sections in the lower hearth and water cooled panels in the side wall and the roof of the vessel, and water is circulated continuously through the panels in a continuous circuit.
The HIsmelt process enables large quantities of molten iron, typically at least 0.5 Mt/a, to be produced by smelting in a single compact vessel.
The HIsarna process is carried out in a smelting apparatus that includes (a) a smelting vessel that includes a smelting chamber and lances for injecting solid feed materials and oxygen-containing gas into the smelting chamber and is adapted to contain a bath of molten metal and slag and (b) a smelt cyclone for pre-treating a metalliferous feed material that is positioned above and communicates directly with the smelting vessel.
The term “smelt cyclone” is understood herein to mean a vessel that typically defines a vertical cylindrical chamber and is constructed so that feed materials supplied to the chamber move in a path around a vertical central axis of the chamber and can withstand high operating temperatures sufficient to at least partially melt metalliferous feed materials.
In one form of the HIsarna process, carbonaceous feed material (typically coal) and optionally flux (typically calcined limestone) are injected into a molten bath in the smelting chamber of the smelting vessel. The carbonaceous material is provided as a source of a reductant and a source of energy. Metalliferous feed material, such as iron ore, optionally blended with flux, is injected into and heated and partially melted and partially reduced in the smelt cyclone. This molten, partly reduced metalliferous material flows downwardly from the smelt cyclone into the molten bath in the smelting vessel and is smelted to molten metal in the bath. Hot reaction gases (typically CO, CO2, H2, and H2O) produced in the molten bath is partially combusted by oxygen-containing gas (typically technical-grade oxygen) in an upper part of the smelting chamber. Heat generated by the post-combustion is transferred to molten droplets in the upper section that fall back into the molten bath to maintain the temperature of the bath. The hot, partially-combusted reaction gases flow upwardly from the smelting chamber and enter the bottom of the smelt cyclone. Oxygen-containing gas (typically technical-grade oxygen) is injected into the smelt cyclone via tuyeres that are arranged in such a way as to generate a cyclonic swirl pattern in a horizontal plane, i.e. about a vertical central axis of the chamber of the smelt cyclone. This injection of oxygen-containing gas leads to further combustion of smelting vessel gases, resulting in very hot (cyclonic) flames. Incoming metalliferous feed material, typically in the form of fines, is injected pneumatically into these flames via tuyeres in the smelt cyclone, resulting in rapid heating and partial melting accompanied by partial reduction (roughly 10-20% reduction). The reduction is due to both thermal decomposition of hematite and the reducing action of CO/H2 in the reaction gases from the smelting chamber. The hot, partially melted metalliferous feed material is thrown outwards onto the walls of the smelt cyclone by cyclonic swirl action and, as described above, flows downwardly into the smelting vessel below for smelting in the smelting chamber of that vessel.
The net effect of the above-described form of the HIsarna process is a two-step countercurrent process. Metalliferous feed material is heated and partially reduced by outgoing reaction gases from the smelting vessel (with oxygen-containing gas addition) and flows downwardly into the smelting vessel and is smelted to molten iron in smelting chamber of the smelting vessel. In a general sense, this countercurrent arrangement increases productivity and energy efficiency.
The HIsmelt and the HIsarna processes include solids injection into molten baths in smelting vessels via water-cooled solids injection lances.
In addition, a key feature of both processes is that the processes operate in smelting vessels that include a smelting chamber for smelting metalliferous material and a forehearth connected to the smelting chamber via a forehearth connection that allows continuous metal product outflow from the vessels. A forehearth operates as a molten metal-filled siphon seal, naturally “spilling” excess molten metal from the smelting vessel as it is produced. This allows the molten metal level in the smelting chamber of the smelting vessel to be known and controlled to within a small tolerance—this is essential for plant safety. Molten metal level must (at all times) be kept at a safe distance below water-cooled elements such as solids injection lances extending into the smelting chamber, otherwise steam explosions become possible. It is for this reason that the forehearth is considered an inherent part of a smelting vessel for the HIsmelt and the HIsarna processes.
The term “forehearth” is understood herein to mean a chamber of a smelting vessel that is open to the atmosphere and is connected to a smelting chamber of the smelting vessel via a passageway (referred to herein as a “forehearth connection”) and, under standard operating conditions, contains molten metal in the chamber, with the forehearth connection being completely filled with molten metal.
Normal start-up for both the HIsmelt and the HIsarna processes in a smelting vessel includes the following steps:
1. Preheating refractories in the lower parts of the (nominally empty) smelting vessel, including the forehearth chamber and the forehearth connection.
2. Pouring externally prepared hot metal into the smelting vessel via the forehearth in such a quantity that the metal level is at least about 100 mm above the top of the forehearth connection.
3. Optionally injecting fuel gas (such as natural gas or LPG) and oxygen-containing gas into the gas space above the metal bath for a period of time to generate heat in the smelting chamber.
4. Commencing and thereafter continuing injection of coal (preferably with flux additions) and oxygen-containing gas, for the purpose of heating the metal charge and initiating slag formation and increasing the amount of slag.
5. Optionally injecting crushed slag and/or slag-forming agents such as silica sand/bauxite plus lime/dolomite flux to further accelerate slag formation.
6. Initiating injection of iron-containing material such as iron ore (together with coal and flux) to commence normal smelting operation.
Practical experience of the applicant has shown that the above start-up sequence, if not carefully controlled, can easily lead to excessively high heat fluxes on water-cooled elements, such as water-cooled panels, in the lower parts of the smelting vessel—typically greater than 500 kW/m2 heat fluxes.
For the purpose of this discussion, the term “lower parts” is understood to mean the exposed water-cooled elements (normally coated with a layer of frozen slag when the plant is in operation) in the bottom 2-2.5 m (vertically) of all water-cooled elements within the smelting vessel when the plant is of “small” industrial size (e.g. HIsmelt 6 m vessel). For a smaller plant (e.g. HIsarna 2.5 m pilot plant) this distance will be proportionally reduced, and may be around 1-1.5 m. Conversely, for a very large plant (e.g. HIsmelt 8 m plant) this distance will increase to about 2.5-3 m.
An “exposed” water-cooled element is understood herein to mean an element which:
(i) has at least 30% of its external surface area that is inside the vessel splashed with molten metal and/or slag when the plant is in normal operation, and
(ii) is cooled internally by convective heat transfer to water in the liquid phase, with the cooling water typically being in the range 10-80° C. and 0-10 bar gauge, and water velocity in the cooling channels exceeding 0.5 m/s.
Depending on the particular design of water-cooled elements in the lower parts of the smelting vessel, heat fluxes in excess of 500 kW/m2 can trip the plant, forcing the start-up sequence to be aborted temporarily. Water-cooled elements can be designed to withstand higher heat fluxes (e.g. 700-800 kW/m2), although this tends to increase cost of the elements. With water-cooled elements designed to withstand heat fluxes in excess of 500 kW/m2 the “window” of operation is larger, but the same overall logic applies.
For the purpose of this discussion the amount of “500 kW/m2” is understood to mean the design maximum heat flux of water-cooled elements in the lower parts of the vessel. It is emphasised that the present invention is not confined to water-cooled elements having a design maximum heat flux of 500 kW/m2. The measurement of these heat fluxes is also understood to exclude short-term (<30 second) (measurement-related) fluctuations, with heat fluxes referred to herein being time-averaged over 30 seconds or more.
If the plant trips as a result of heat fluxes in excess of the design maximum heat flux, the result is a delay leading to unplanned cooling of metal in the smelting vessel—in particular, cooling of metal in the forehearth connection. If the metal cools beyond a certain point it becomes necessary to end-tap the vessel to avoid a frozen forehearth connection. The entire start-up is thus aborted and the whole start-up sequence must start again (at significant cost and lost production time).
In general, the period during which water-cooled elements in the lower parts of the smelting vessel are subject to possible high heat flux exposure is limited to the time needed to establish a slag layer deep enough to (predominantly) slag-splash and/or slag-submerge the bottom row of water-cooled elements. Once slag-splashed or slag-submerged, these water-cooled elements form moderately thick (>10 mm) slag freeze-layers and heat fluxes drop to significantly lower levels (typically <200-250 kW/m2).
The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.